Omega is an accurate all-weather, global electronic navigational system. Omega supplies the navigational information regardless of weather or time of day, by providing very low frequency continuous pulsed wave transmissions to ships in any sea, land, or air vehicles anywhere. This system is available to both military and civilian users and all nations. In this connection, the system is being installed on all U.S. Naval surface ships capable of independent operation as a general purpose all weather electronic aid to navigation. Fishing boats and merchant ships have also been equipped with this system.
The Omega navigational system utilizes phase-difference measurement techniques for navigation. This system contemplates eight transmitter locations such that a ship or aircraft anywhere in the world will be within range of at least 3 and probably 4 transmitters. Each station transmits Omega signals on a time-share basis, when no other station is on the air, and time shared frequencies in the 10 to 14 KHz band. In practice, the fundamental measurement in Omega is the phase of the 10.2 KHz signal from each of the several stations. The 10.2 KHz transmission at each station lasts approximately 1 second and is repeated each 10 seconds.
The receiver measures the phase of the 10.2 KHz signal from a selected transmitter in comparison with the signal from a second station. The phase difference establishes a line of position (LOP) on an Omega chart. Another pair of stations provide a second line of position, establishing a fix.
Omega receivers in use today are simple to operate with a numerical readout displaying continuously the Omega coordinates of the vehicle's position based on the phase difference of received signals. Omega receiving systems model numbers AN/SRN-14 and AN/BRN-4 were manufactured by Avionics Division ITT, Nutley, N.J. 07710.
In the Omega navigational system, signals are transmitted in a format that is repeated at 10 second intervals. All navigational signals are transmitted during 8 segments that are 0.9 to 1.2 seconds long with 0.2 second spacing. The 10 second format with eight 0.2 second OFF intervals is generated in an Omega receiver. Successful search consists of synchronizing the format generated in the receiver with that of the received signals. However, the synchronization is rendered difficult because of the following factors:
1. Low cost Omega receivers operate on 10.2 KHz only.
2. Only some of the 8 stations will be received at any time.
3. Noise and weak signals will give false indications of signal absence.
4. The response of narrow-band filters to noise will cause false indications of signal presence.
5. The Omega segment format has a poor autocorrelational factor, rendering the received signal pattern ambiguous under noise conditions.
Thus, the search problems may be divided into two areas, namely, (i) satisfactory detection of Omega RF signals under adverse conditions, and (ii) a technique for relating the receiver segment timing with the detected signals.
In Omega navigational receiving systems heretofore proposed, synchronization has been achieved manually. Within the recent past a computer controlled correlation technique for achieving synchronization has also been proposed. Manual synchronization is time consuming and inaccurate. The correlation technique requires the use of an expensive general purpose computer.